Calibration Procedure; Calibration Points; Measuring The Set-Point Error; Computing R0 And Alpha - Fluke Calibration 6020 Series User Manual

11

Calibration Procedure

In some instances the user may want to calibrate the bath to improve the tem-
perature set-point accuracy. Calibration is done by adjusting the controller
probe calibration constants R0 and ALPHA so that the temperature of the bath
as measured with a standard thermometer agrees more closely with the bath
set-point. The thermometer used must be able to measure the bath fluid temper-
ature with higher accuracy than the desired accuracy of the bath. By using a
good thermometer and carefully following procedure the bath can be calibrated
to an accuracy of better than 0.02°C over a range of 100 degrees.
11.1

Calibration Points

In calibrating the bath R0 and ALPHA are adjusted to minimize the set-point
error at each of two different bath temperatures. Any two reasonably separated
bath temperatures may be used for the calibration however best results will be
obtained when using bath temperatures which are just within the most useful
operating range of the bath. The farther apart the calibration temperatures the
larger will be the calibrated temperature range but the calibration error will also
be greater over the range. If for instance 50°C and 150°C are chosen as the cali-
bration temperatures then the bath may achieve an accuracy of say ±0.03°C
over the range 40 to 160°C. Choosing 80°C and 120°C may allow the bath to
have a better accuracy of maybe ±0.01°C over the range 75 to 125°C but out-
side that range the accuracy may be only ±0.05°C.
11.2

Measuring the Set-point Error

The first step in the calibration procedure is to measure the temperature errors
(including sign) at the two calibration temperatures. First set the bath to the
lower set-point, t
to stabilize at that temperature. Check the bath stability with the thermometer.
When both the bath and the thermometer have stabilized, measure the bath tem-
perature with the thermometer and compute the temperature error err
the actual bath temperature minus the set-point temperature. For example, if the
bath is set for a lower set-point of t
temperature of 49.7°C then the error is –0.3°C.
Next, set the bath for the upper set-point, t
bath temperature and compute the error, err
bath was set for 150°C and the thermometer measured 150.1°C giving an error
of +0.1°C.
11.3

Computing R0 and ALPHA

Before computing the new values for R0 and ALPHA the current values must
be known. The values may be found by either accessing the probe calibration
menu from the controller panel or by inquiring through the digital interface.
. Wait for the bath to reach the set-point and allow 15 minutes
L
=50°C and the bath reaches a measured
L
11 Calibration Procedure
, and after stabilizing measure the
H
. For our example, suppose the
H
Calibration Points
which is
L
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